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Old 08-08-2012, 05:58 PM   #11
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Default Re: Rear Window Air Conditioner

The chart shows resting voltages. It illustrates battery capacity. It is unrelated to voltage under a load. Take the load off and the batter voltage (and therefore capacity) will be higher.
The source is here: http://www.windsun.com/Batteries/Batter ... 20Capacity Great info.

Your setup seems ok but the 6 gauge wire is small. I went small on mine also but it is 4 gauge with a 1000 watt inverter. That would not cost too much to change.
The 250 amp initial surge on a 4' long round-trip circuit with 6 gauge wiring run causes a .49 volt drop on a starting voltage of 12.7 volts (3.86% drop). The voltage drop with 2/0 (00) gauge would only be .19v (1.5%)

I picked 250 amps DC as the surge because of the test that other guy did that showed 22.6 amps AC inrush. 22.6 x 110v AC = 2500 watts (approx). Divide that by 10 for the DC amp hr draw.
I picked 10 as the number to divide by (instead of 12v) because my video and ammeter photo pretty much demonstrates that. It showed the inverter output as 450 watt AC and a 43.5 amp hr DC draw. That's roughly a 10x multiple. The loss is caused by wiring and inverter inefficiencies. Inverters are 90% efficient or thereabouts.

Air conditioner run-time is going to be limited. 210 a/h x 50% = 105 ah. I'd guess that you could get two hours of actual cooling (compressor + fan time) then your batteries would very much need to be recharged. I'm roughly factoring in a "penalty". Peukert's law
Quote:
If the battery is discharged in a shorter time, with a higher current, the delivered capacity is less.
The 105 amp hr rating on your batteries may have been calculated at the 20 hr rate meaning that after a 20 hour test the ending voltage was 10.5 volts and the battery delivered 105 amp hrs.

Lot's of inverters shut off at 10.5 volts but that is not 10.5 volts at rest. It is 10.5 volts under a load. They are two very different measurements. 10.5 volts at rest is most likely an unrecoverable battery. Good for its trade in recycling value.
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Old 08-08-2012, 06:59 PM   #12
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Default Re: Rear Window Air Conditioner

Hi again Marko,
Quote:
Lot's of inverters shut off at 10.5 volts but that is not 10.5 volts at rest. It is 10.5 volts under a load. They are two very different measurements. 10.5 volts at rest is most likely an unrecoverable battery. Good for its trade in recycling value
Marko, there is an error in the info i gave about the batteries - they are rated at 225 a/h @ 20 hr rate, and 180 a/h @ 6 hr rate.

So if i have for example 6 batteries (6 volt, 180 a/h, 6hr rate), that will give me total of 540 a/h @ 12v (again, 6hr rate) - this is in resting mode.
So actually i do not have 540 amps at load mode, since under load there is a drop in the voltage, which means the voltage is closer to the shut off point of the inverter.

My question is: According to your video, fully charged batteries will start at 12v under load, while at resting mode they will start at 12.7v - does that mean that it will get to the shut off voltage point much faster - therefore much less time to rely on the batteries?

Is there any way you could estimate the difference in a/h i could rely on the batteries under load, if i know the resting a/h capacity?

Thanks again,
Moses
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Old 08-08-2012, 08:01 PM   #13
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Default Re: Rear Window Air Conditioner

You are doing that math right. Here's an explanation for anyone who doesn't understand series and parallel battery connections:

Quote:
In the SERIES CONNECTION, batteries of like voltage and Amp-Hour capacity are connected to increase the Voltage of the bank. The positive terminal of the first battery is connected to the negative terminal of the second battery and so on, until the desired voltage is reached. The final Voltage is the sum of all battery voltages added together while the final Amp-Hours remains unchanged. The bank's Voltage increases while its Amp-Hours, Cranking Performance and Reserve Capacity remain unchanged.
Quote:
In the PARALLEL CONNECTION, batteries of like voltages and capacities are connected to increase the capacity of the bank. The positive terminals of all batteries are connected together, or to a common conductor, and all negative terminals are connected in the same manner. The final voltage remains unchanged while the capacity of the bank is the sum of the capacities of the individual batteries of this connection. Amp-Hours Cranking Performance and Reserve Capacity increases while Voltage does not.
------------------------------------------------------------------------------------------------------------

Quote:
Originally Posted by pjc188
So actually i do not have 540 amps at load mode, since under load there is a drop in the voltage, which means the voltage is closer to the shut off point of the inverter.
The voltage drop will vary depending on the load.

Quote:
Originally Posted by pjc188
My question is: According to your video, fully charged batteries will start at 12v under load, while at resting mode they will start at 12.7v - does that mean that it will get to the shut off voltage point much faster - therefore much less time to rely on the batteries?
Again, The voltage drop will vary depending on the load. Mine were under a 43.5 ah load. And, my batteries are fairly new so they still get to 12.7v when fully charged. The numbers will be different for every system.

Quote:
Originally Posted by pjc188
Is there any way you could estimate the difference in a/h i could rely on the batteries under load, if i know the resting a/h capacity?
Here's my guess. 6 batteries will share the load very well. I'll think of yours as 3 12v 180ah batteries. Each pair can deliver 30ah for 6 hours. So combined they're rated to be able to deliver 90ah for six hours (approximately - they might start higher and finish lower).
Lets assume you are comfortable drawing them down to 50% so roughly 270ah available at the combined 90ah rate. You'll be drawing them down at approx half that rate (my second test showed 43.5ah draw) so the batteries should easily perform to their specified rating. In theory, you'll should get 6 hours of cooling (compressor + fan) time.

The compressor will cycle on and off so you probably won't ever use 270ah overnight unless it is very hot and the air conditioner runs continuously. But, if you did use 270ah, then you have a big problem - how do you replace the 270ah you took out? You'll probably have to drive 6 or 7 hours and that might get you back to 90% charged. That might be very hard on a regular alternator.

I still see it this way:
Quote:
The air conditioner compressor will cycle on and off as needed depending on the temperature. If I set the temperature on the A/C at 72F I could get up to two hours compressor cooling time which might equal 3 to 4 hours of a cool rear section of the van if I further block off that rear area. That might make the difference of a good nights sleep or not. This will really only be practical if I'll be driving for a few hours the next day or will soon have access to grid power to recharge the battery bank.
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Old 08-08-2012, 11:54 PM   #14
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Default Re: Running small Air Conditioner on an inverter

Some new data

There are 4 batteries in my van; 2 house and 2 starting for the diesel engine. I have a marine style multi-battery switch that lets me choose #1. the house battery or #2. the engine battery or #3. Both banks combined or #4. All off. I can momentarily combine both banks and it lets me start the and run the air conditioner with the a/c on cool mode. It is a "make before break" switch so I can then switch back to #1. house battery only without interrupting the current flow. The current drawn from the engine batteries would be measured in amp-seconds; too small to matter. The inverter shows 12.2 volts when all 4 batteries are paralleled by the multi-battery switch and running the air conditioner on the inverter.

There's more
Started the van. House and Engine battery banks get paralleled by the isolator. Additionally, I then set the multi-battery switch to "Both".

With the van engine running at idle, my Scangauge reports .08 GPH (gallons per hour) fuel consumption and 624 RPM.

With the van engine running at idle and the rear window air conditioner running off my inverter, my Scangauge reports .11 GPH (gallons per hour) fuel consumption and 634 RPM. 13.5 volts reported by the inverter.

With the van engine running at idle and the rear window air conditioner running off my inverter and the van dash air on "max air" blowing cold, my Scangauge reports .45 GPH (gallons per hour) fuel consumption and 651 RPM. Voltage shown by the inverter is still 13.5 volts.

There was no input from the solar panels for this test.
The alternator, at idle, can carry the load of both the rear window air conditioner and the vans front dash air conditioner. Combining the two air conditioners will allow more rapid cooling. Might be useful.

Daytime running lights come on when the van motor is running. Turning them off in my van is easy, just press the emergency brake down. Even just one click will turn off the daytime running lights. Useful if I'm parked.

I don't think the Fedders 5200 btu air conditioner in my van exchanges air. I think it just recirculates the air in the van.
From the specifications:

I haven't slept with it running but maybe I should leave the Fantastic fan open just a bit if i ever do.

Was thinking about C0 (carbon monoxide) with the van motor running. I do have a new from last year C0 detector in the van.

Anyway that's the new data, I'm not suggesting it as a solution for anyone. It really needs to be well thought out by anyone trying something like this.
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Old 08-09-2012, 05:29 AM   #15
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Default Re: Running small Air Conditioner on an inverter

Hi Marko,
Quote:
Lets assume you are comfortable drawing them down to 50% so roughly 270ah available at the combined 90ah rate. You'll be drawing them down at approx half that rate (my second test showed 43.5ah draw) so the batteries should easily perform to their specified rating. In theory, you'll should get 6 hours of cooling (compressor + fan) time.
So If i'm going to use around 270 a/h, so batteries will be @ 50% discharge - under load, this condition will not shut off the inverter?
In other words - as far as i understood, we will not know the voltage after approx. 6 hrs of cool mode.

I there any way to be more precise on this issue?

Also
Quote:
...they might start higher and finish lower
- did you mean the battery voltage?

Thanks again,
Moses
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Old 08-09-2012, 11:24 AM   #16
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Default Re: Running small Air Conditioner on an inverter

I don't think the voltage will go low enough to shut off your inverter. You'll only be using 50% of the battery bank capacity. The wiring and temperature will have an affect. You won't really know until you try.
Deep cycle batteries can be discharged 80%. Limiting that to 50% is easier on the batteries.

Quote:
they might start higher and finish lower
- I was referring to amps and how they calculate the 6 hr rating. I don't know how they do the calculation.

I plugged in your numbers here: - excel spreadsheet - http://www.smartgauge.co.uk/calcs/peukert.xls


The calculator above shows 14 hours to completely discharged at 45ah. So approx 7 hours to 50%.

I'm very curious to know how you plan to recharge the batteries.
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Old 08-09-2012, 02:11 PM   #17
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Default Re: Running small Air Conditioner on an inverter

Hi Marko,
Thanks again for your reply - super appreciated

About ccharging my batteries: this will be done by mainly solar power, and when it's not enough, by small generator.
I am planning to make a 48v system in the van instead of standard 12v, and here is why:
I calculated that i will be able to put 5 solar panels on my van's top - they are approx. 3 feet wide each.
Each provides on a sunny day about 7-8 a/h per hour (about 5 hrs of the day the sun is highly effective).
So 5 of these panels times 7 a/h is 35 a/h per hour x 5 hrs of sun = 175 a/h.
The windoe AC we want to run takes around 43.5 to 50 a/h on 12v - so we have a little more than 3 hrs to run the AC with the a/h the solar system will provide - 175 a/h (provided by solar panels) divided by 50 a/h (consumed hourly by the AC) will give roughly 3.5 hrs of use only.
Now - let me explain why i'll use 48v system - instead of 12v.
The 48v panels are almost the same size as the 12v.
48v panel, on a sunny day, provides about 5 a/h.
5 panels will produce around 25a/h.
At 48v, the AC will use 10-12 a/h, instead of 43.5-45 a/h on 12v.
So now we have a system that provides us 25 a/h per hour, times 5 hours of a full sun (5 hours full sun on average) - 25a/h x 5 hrs = 125 a/h.
So if the AC will use even 15 a/h, the system at least, will provide enough power to run that AC for at lease 8 hrs.
Here is the calculatio: 125 a/h (provided by solar panels) divided by 15 a/h (consumed hourly by the AC) will give roughly 8.33 hrs of use.
That's, in my opinion - not bad at all.

Please verify and let me know what you think.
Thanks again,
Moses
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Old 08-09-2012, 03:37 PM   #18
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Default Re: Running small Air Conditioner on an inverter

It's clever and you're thinking "outside the box" for sure. I doubt that using roughly the same amount of batteries could yield such dramatically different results in actual use. And you have the complexities of combining typical 12v RV items (pump, lights, fans, furnace, fridge controls, C0 and propane detectors, etc.) and a 12v van alternator with a 48v system with 48v inverter.

So that would be 8 x 6v batteries in series but still only 225 a/h capacity @ 20 hr rate (I don't know if the voltage affects the rating). Only 112 a/h useable if discharged to 50%. Divide by 12 hr air conditioner use = approx 9.3 hours on battery.

They might recharge then next day - I'm not sure - because the 25a/h combined from the panels would equal only 3.125 a/h per battery. Closer to a "trickle" charge than to a "bulk" charge. It can take a long time to get a battery 100% charged.

That solar array would more than power the air conditioner during a sunny day but you'd probably be outside anyway. Parking in the shade, rainy days, winter sun, performance loss from panel heat build up all need to be factored in. 8 batterys would take up a lot of space and you need to access them for maintenance. 8 AGM's would be $$$$.

The Peukert calculator shows 8 x 6v 225 a/h capacity @ 20 hr rate batteries in a series parallel configuration to give 12v would let you draw 45ah for 10 hours to get to 50% discharged.

12 volt series/parallel


I love solar and have had it on 4 RV's but it hard to beat a generator for powering an Air Conditioner.
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Old 08-09-2012, 03:53 PM   #19
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Default Re: Running small Air Conditioner on an inverter

Quote:
They might recharge then next day - I'm not sure - because the 25a/h combined from the panels would equal only 3.125 a/h per battery. Closer to a "trickle" charge than to a "bulk" charge. It can take a long time to get a battery 100% charged.
Hi Marko,

The 25 a/h is per hour - if we average 5 hrs full sun, we'll have 125 a/h.
8 x 6v will give us a battery bank that the system will see as one large 48v battery.
If the system will push 125 a/h to a 48v "battery", i think it will bulk charge and trickle charge - even with losses, you'll still get between 7-8 hours of comp+fan.
Assuming the AC will cycle on and off - looks like pretty good 9-10 hours of AC when reaching 50 batt discharge.

Oh, another thing i forgot to mention is that even in the hours that the sun is not at her peak (early morning, early evening) we can still get extra amps.

Anyway, the only problem is the cost...
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Old 08-09-2012, 05:07 PM   #20
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Default Re: Running small Air Conditioner on an inverter

Yes, 25ah (per hour) from panels divided by 8 batteries = 3.125ah (per hour) per individual battery. It just seems low to me. I don't have any experience with 48v systems though.

Yes, the cost................
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